STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF PEDERSON QUARRY NEAR HUMBOLDT, IOWA

Pederson Quarry, located east of Humboldt, Iowa, is situated in the "Humboldt Oolite" of the Gilmore City Formation. This formation belongs to the Osagean Stage of the Mississippian System. The section exposed at the quarry is dominated by medium to very-fine grained oolitic and bioclastic grainstone and capped by crystalline dolomite. Fossil material present includes solitary rugose corals, gastropods, brachiopods, crinoid debris, and microfossil remains. Sedimentary structures include planar fenestral birdseyes and pisoids. Secondary processes evident include faulting (slickensides), pressure-solution activity (stylolites), and infilling of fractures and vugs with calcite. In modern analogs, fenestral birdseyes are usually restricted to the supratidal zone, occasionally forming in the intertidal zone. The birdseyes likely formed when gas bubbles became trapped in sediment, with subsequent compaction and shrinkage causing the planar shape. Birdseyes are also a reliable indicator of early lithification. The pisolite layers occurring near the top of the section are more problematic. Genesis of pisoids has been a controversial topic for years; hypotheses on the formation of pisoids have included algal coating, rolling by wave action, lithoclast aggregation, and formation in place. Past workers at the Humbolt locality have postulated that the pisoids were precipitated from seawater in the high intertidal and supratidal zones. Stratigraphic and faunal analysis suggest this portion of the "Humboldt Oolite" represents a shallowing upward carbonate sequence restricted to the intertidal and supratidal zones.